The purpose of this project is to characterize the metabolism of vitamin D by T-cell lymphocytes in vitro with particular attention to the production of 1,25-dihydroxyvitamin D (1,25(OH)2D), the most active metabolite of vitamin D. Preliminary results from the Principal Investigator's laboratory indicate that HTLV-transformed cord blood lymphocytes have the capacity to convert 25(OH)D to a more polar metabolite which coelutes with authentic 1,25(OH)2D using two separate high performance liquid chromatography systems and stimulates bone resorption in vitro. Identification of this metabolite as 1,25(OH)2D by gas chromatography/mass spectrometry suggest that these transformed lymphocytes have 1-alpha-hydroxylase activity. Since 1,25(OH)2D is a potent stimulator of osteoclastic bone resorption, increased production of 1,25(OH)2D or other vitamin D metabolites by lymphoma cells may play a role in the pathogenesis of the increased osteoclastic bone resorption and hypercalcemia which occur in this disease. Accordingly, the purpose of this study is to use HTLV-transformed lymphocytes as an in vitro model to study the metabolism of vitamin D by lymphocytes in order to: 1. Characterize the metabolites of vitamin D which are produced by HTLV transformed T-lymphocytes with particular attention to the production of 1,25(OH)2D. 2. Examine the metabolism of vitamin D by peripheral lymphocytes in patients with other disease states characterized by abnormal lymphocyte function and altered bone and mineral metabolism. 3. Determine if activated normal lymphocytes have 1-alpha-hydroxylase activity. 4. Characterize the 1-alpha-hydroxylase activity in normal and abnormal lymphocytes to see if it is regulated by the same factors which regulate renal 1-alpha-hydroxylase activity. These studies will determine whether or not the ability to produce 1,25(OH)2D is a general property of all activated lymphocytes or is a unique property of lymphocytes infected with the HTLV virus. Moreover, characterization of this 1-alpha-hydroxylase activity and identification of those factors which regulate lymphocyte 1-alpha-hydroxylase activity will provide important information concerning factors that control vitamin D metabolism both in normal individuals and disease states characterized by abnormal bone and mineral metabolism. These results will also form the basis of studies designed to elucidate the molecular mechanisms responsible for the activation of 1-alpha-hydroxylase activity in lymphocytes.